1. Background of the Invention
The present invention relates generally to marine seismic surveying and, more particularly, to a method and apparatus for reducing the signal noise from vertical movement in a dual sensor towed streamer cable caused by vibrations in the stress members of the streamer.
2. Description of Related Art
Seismic surveying is a method for exploring subterranean formation layers in the earth. An acoustic source generates seismic waves, which insonify the formation layers. Differences in acoustic impedance of adjacent formation layers cause a portion of the seismic waves to reflect from the interfaces between the formation layers. Acoustic impedance varies across formation layers since it is the product of seismic wave velocity and rock density. Seismic sensors detect the seismic waves reflected upward from the formation interfaces and record wave amplitude versus time of arrival as electrical signals for later analysis regarding the locations of the formation interfaces.
Marine seismic surveying is seismic surveying for formation layers in parts of the earth located beneath bodies of water. An acoustic source placed in the water, such as an airgun, generates the seismic waves which insonify the subterranean formation layers. Seismic sensors, typically arrayed at intervals along a streamer cable towed in the water behind a vessel, detect the reflected seismic waves. Marine seismic surveying typically uses pressure sensors, such as hydrophones, to detect changes in water pressure caused by seismic compression and rarefaction waves propagating through the water. The pressure sensors detect the primary pressure waves traveling upward in the water after reflection from the formation interfaces in the earth below the water. The pressure sensors also detect secondary pressure waves traveling downward in the water after a portion of the primary waves traveling upward reflect down from the water surface above. The air-water interface at the water surface has a large contrast in acoustic impedance which causes a large downward reflection. Secondary reflections are unwanted ghost waves, a type of noise in the seismic signal.
The water-earth interface at the water bottom may also have a large contrast in acoustic impedance. Thus, the downward-traveling secondary reflections from the water surface may reflect back upward again from the water bottom. Thus secondary reflections may continue to reverberate through the water column from surface to bottom and back. Water column reverberation is a serious source of signal noise obscuring the primary reflections carrying the sought-after information concerning the subsurface formation layers.
FIG. 1 shows a diagrammatic view of marine seismic surveying employing a seismic streamer cable, generally designated as 100. A ship 102 tows a seismic streamer 104 through a body of water 106. The seismic streamer 104 contains a plurality of sensors 108. Subterranean substrata, such as 110 and 112, to be explored, are located in the earth 114 beneath the body of water 106. Interfaces, such as 116, separate the substrata. An acoustic source 118, such as an air gun, creates seismic waves in the water 106. A portion of the seismic waves travel downward along ray paths 120 through the water 106 toward the earth 114. A portion of the downward-traveling seismic waves reflect upward from an interface, such as interface 116 between substrata 110 and 112. The reflected, upward-traveling seismic waves are primary reflections from the formation layers. The primary reflections travel upward along ray paths 122, a portion of which intersect the towed streamer 104. Sensors 108 deployed in the towed streamer 104 detect the primary reflections. The primary reflections travel past the towed streamer 104 and continue along ray paths 122 upward toward the air-water interface 124 at the surface of the body of water 106. A portion of the seismic waves comprising the primary reflections reflect downward from the air-water interface 124. The twice-reflected, downward-traveling seismic waves are secondary reflections from the water surface. The secondary reflections travel downward along ray paths 126, a portion of which intersect the towed streamer 104. The sensors 108 deployed in the towed streamer 104 detect the secondary reflections from the air-water interface 124.
The towed streamer 104 contains a plurality of sensors 108. Towed streamers 104 typically carry pressure sensors, such as hydrophones, which will be described below in FIG. 2. Dual sensor towed streamers 104 carry pairs of pressure sensors and motion sensors, such as geophones or accelerometers. The present invention adds a third sensor, a noise reference sensor, which will be described below in FIG. 3. The third sensor is a variant of the prior art pressure sensor.
FIG. 2 shows a diagram of a pressure sensor 200, an acceleration-canceling hydrophone, typically used in a towed streamer. The pressure sensor 200 typically comprises a housing 202 having a first end and a second end, a first element 204 mounted at the first end of the housing 202, and a second element 206 mounted at the second end of the housing 202. The first element 204 is mounted parallel to the second element 206. The housing 202 is typically made of brass and the first and second elements 204, 206 are typically made of piezoelectric crystal. A first pair of electric wires 208, 210 attaches to the opposing faces of the first element 204 and a second pair of electric wires 212, 214 attaches to the opposing faces of the second element 206. The arrows in FIG. 2 show the relative polarities of the connections.
FIGS. 3a-3d show conceptual diagrams of an acceleration canceling hydrophone 300 subject to accelerations and passing seismic waves. The electric wires 308, 310, 312 and 314 are connected so that flexures of the elements 304, 306 such as shown in FIGS. 3a and 3b generate output voltages which add, resulting in a nonzero signal. The pressure manifestation of a compression seismic wave propagating past the pressure sensor 300 causes the flexure of the elements 304, 306 shown in FIG. 3a, while the pressure manifestation of a rarefaction seismic wave propagating past the pressure sensor 300 causes the flexure of the elements 304, 306 shown in FIG. 3b. This flexures of the elements 304, 306 as shown in FIGS. 3c and 3d generate output voltages which cancel, resulting in a substantially zero signal. Upward motion of the pressure sensor 300 through a fluid causes the flexure of the elements 304 and 306 shown in FIG. 3d. The above-described con figuration accomplishes the acceleration canceling property of the typical marine hydrophone 300. The polarity indications on FIGS. 3(a)-3(d) show the relative polarities of signals generated for the flexure of elements 304, 306 as shown.
Marine seismic surveying also uses motion sensors, detecting particle velocity or acceleration, in addition to pressure sensors. Motion sensors typically used in marine seismic surveying are geophones and accelerometers. Motion sensors detect the vertical velocity or acceleration of water particles accompanying seismic waves propagating past the sensors. Thus motion sensors detect primary and secondary reflections, just as pressure sensors do. Proper combination of the signals from pressure sensors and motion sensors can lead to a reduction of secondary reflections from the water surface in the seismic signal. The air-water interface causes a reverse in polarity in the downward reflected pressure wave, since the acoustic impedance of the water exceeds the acoustic impedance of the air. Thus pressure sensors detect a reverse in phase polarity for the secondary reflections from the water surface. The air-water interface does not cause a reverse in polarity in the vertical motion wave. Thus motion sensors do not sense a reverse in phase polarity for the secondary reflections from the water surface. Pressure and motion detectors sense upward traveling primary reflections with the same polarity while sensing downward traveling secondary reflections with opposite polarity. Therefore, combining the signals from pressure and motion detectors enhances the desired primary reflections and reduces the undesired secondary reflections from the water surface.
Combining the signals from pressure sensors with the signals from motion sensors in a dual sensor towed streamer in an effort to reduce the effects of secondary reflections from the water surface is well known in the prior art. Berni discloses methods for the combination of signals from different sensors in three patents. U.S. Pat. No. 4,345,473 teaches combining the signal from a vertical component accelerometer with the signal from a hydrophone to cancel surface reflected waves in marine surveys. U.S. Pat. No. 4,437,175 teaches combining the signal of a hydrophone with the signal of an accelerometer which has passed through a integrator and a high-pass filter. U.S. Pat. No. 4,520,467 teaches combining the filtered signals of a motion sensor and a pressure sensor in proportion to the signal-to-noise ratios of the sensors. All three Berni patents address the problem of reducing unwanted secondary reflections from the water surface by means of dual sensors in a towed streamer, but merely mention the accompanying problem of insulating or filtering out the noise generated by the movement of the streamer itself. Removing the undesired secondary reflections from the desired primary reflections is not effective if streamer vibration noise still remains to obscure the seismic signal. Signal noise caused by streamer vibration is one of the problems encountered in employing dual sensor towed streamers to solve the problem of secondary reflections from the water surface.
One of the particular problems with previous attempts to implement a dual sensor towed streamer has been the high level of undesired vertical motion caused by vibrations in the stress members of the towed streamer. A towed streamer is typically ballasted to be neutrally buoyant. Thus a geophone deployed in the streamer generates output signals proportional to the vertical particle velocity of the seismic reflection waves in the water. Unfortunately, the geophone also generates signals proportional to the vertical velocity of the streamer itself, as caused by vibrations of the stress members of the streamer. The geophone is detecting and recording the primary and secondary reflection waves needed to combine with the hydrophone signal to reduce the effect of secondary reflections from the water surface. However, the geophone is also detecting and recording further noise in the form of streamer motion. The vibrations of the streamer stress members add obscuring noise to the geophone data. Thus, the mere combination of a pressure wave signal from a hydrophone with a particle motion signal from a geophone may severely degrade the signal-to-noise ratio in the seismic frequency band.
Pavey (U.S. Pat. No. 3,282,293) discloses a device in which an attempt is made to have a velocity sensor that is sensitive only to vibrations of the cable and not to the velocity of motion of the water. The effectiveness of this device is greatly reduced due to the use of a copper coil having a significant inertial mass as part of the sensor.
The present invention is directed toward improving the effectiveness of the dual sensor towed streamer by reducing the signal noise coming from vertical motion caused by vibrations in the stress members of the streamer.
The present invention is a method for reducing the signal noise from vertical movement in a dual sensor towed streamer cable caused by vibrations in the stress members of the streamer. A seismic streamer is towed in a body of water while deploying three (3) sensors in close proximity to each other. The first sensor generates a first signal indicative of pressure of the water, the second sensor generates a second signal indicative of vertical movement of water particles and vertical motion of the streamer in the water, and the third sensor generates a third signal indicative of vertical movement of the streamer relative to the water, i.e., streamer noise. A seismic signal is generated in the water and the resulting signals are recorded from each of the three (3) sensors simultaneously during time intervals following the generation of the seismic signal. The second signal is combined with the third signal to generate a fourth signal that is a measurement of vertical movement with the streamer noise attenuated. The first signal is combined with the fourth signal to give a ghost free seismic reflection signal.